-------------------------------------------------------- ABSTRACT----------------------------------------------------------- This work involves the revision of the Rainfall Intensity Duration Frequency (IDF) Curves for the city of Kumasi. Annual Maximum Rainfall depths of various durations over twenty-two years were obtained from the Ghana Meteorological Services. The data set was then subjected to frequency analysis to determine the distribution of which best characterize the data set. The annual maximum series were found to be drawn from the Gumbel distribution whose parameters were computed by fitting the statistics to the data. The Chi-square test and the Kolmogorov-Smirnov test prove the appropriateness of the fitting. Since the data available was only 22 years, IDF values for return periods higher than 22 years were obtained using frequency factors. The IDF estimates resulting from this work have been compared with the existing IDF curves prepared by J.B Danquah. The results show that for shorter durations (12 min and 24 min), the new IDF Curves give higher intensities for the same return period; the percentage increase ranges between 2% and 25%, whiles for longer durations (42min, 1 hr, 2hr, 3hr, 6hr, 12hr and 24 hr), the new IDF Curves give lower intensities for the same return period with the percentage decrease ranging between 3% and 49% when compared with the existing J.B Danquah IDF Curves. This might be as a result of low precipitation trends for shorter durations and high precipitation trends for longer durations in 1970s and before. These therefore call for the revision and updating of the existing IDF Curves for all the major cities and towns in Ghana to take into account the effect of climate change. Keywords: IDF Curves, Gumbel Distribution, Annual Maximum Series, Climate Change, Rainfall Intensity---------------------------------------------------------------------------------------------------------------------------------------Date of Submission: 18 January 2017 Date of Accepted: 05 February 2017----------------------------------------------------------------------------------------------------------------------------- ---------

I. INTRODUCTIONThe design of any civil engineering infrastructure requires a comprehensive understanding of the intendedpurpose of the structure and the physical environment in which it would operate. In the management of stormwater in a municipality, the dimensions of various components of the infrastructure system are based on the returnperiod of heavy rainfall events. This information is often expressed as Intensity-Duration-Frequency (IDF) curvesobtained from a statistical study of extreme events. The IDF curves are empirical relationships that relate therainfall intensity, the duration and the return period in a given catchment area.The establishment of such relationships was done as early as 1932 by Chow [1] and Dupont and Allen [2]. Sincethen, a lot of these curves have been developed in many parts of the world. Example Chen [3] developed a simplemethod to obtain a generalised rainfall IDF formula for any location in the United States using three isopluvialmaps of the U.S Weather Bureau. Yu et al. [4] also developed regional IDF relations for non-recording sites basedon scaling theory using the hypothesis of piecewise simple scaling combined with the Gumbel distribution. InAfrica, Mohymont et al. [5] assessed IDF-curves for precipitation for three stations in Central Africa and proposedmore physically based models for the IDF-curves, Oyebande [6] established IDF curves for Nigeria, similarly,Dankwa [7] developed IDF curves for various towns and cities in Ghana.The normal balance of the earths hydrological cycle has been altered due to the changes in temperature andprecipitation patterns. Projections from climate models suggest that the probability of occurrence of intenserainfall in future will increase due to the increase in green house gas emissions [8]. The design standards at presentare based on historical climate information. Under a changing climate, it has become a priority for municipalitiesto search for appropriate procedures in the planning and management of water resources infrastructure to deal withchanging climatic conditions [9].

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Revision of the Rainfall Intensity Duration Frequency Curves for the City of Kumasi-Ghana Journal ..Rainfall intensity is an important parameter for the determination of peak flow at a point in a catchment, which isused for the design of drainage structures. The IDF curves developed by Dankwa [7] is what is used in Ghana forthe design of drainage structures in various cities. These IDF curves were developed based on the rainfall dataavailable at that time. One of the main assumptions in the creation of these curves was that the rainfall data wasstationary. That is, climatic trends and variability have negligible effects on the curves. But as has been proved inrecent history, climatic variability and trends do exist and their effects on precipitation have not been negligible.Climate change has lead to droughts and floods around the world, and long term trends in rainfall, both increasesand decreases, have been seen in different parts of the world including Ghana. There is therefore a possibility thatthe rainfall intensities used by Dankwa [7] for the development of the IDF curves might have changed. The IDFcurves developed by Dankwa [7] have not been revised or updated, however, drainage engineers in Ghana stillrely on these curves for their designs. This might lead to over-design or under-design due to the changes inclimatic conditions that might have occurred after 1974. Over-designing leads to economic losses as biggerstructures are designed whereas under-designing leads to drains of inadequate capacity leading to increasedincidents of flooding. In both cases economic losses are incurred.To reduce economic losses due to over design or under design of drainage structures that might occur using IDFcurves developed by Dankwa [7], there is therefore the need for its revision. The main objective of this projecttherefore is to develop IDF curves for Kumasi based on additional available rainfall information and verify thevalidity of the existing IDF curves developed by Dankwa [7]. It is hoped that this research work will catapultfurther research works into revision and updating of the existing IDF Curves for all the major cities and towns inGhana to take into account the effect of climate change.

II. MATERIALS AND METHODS

2.1 Study AreaKumasi (Fig. 1) is a city in southern central Ghana's Ashanti region. Kumasi is the second largest city in Ghanaalso the capital of the Ashanti region.

Figure1. Map of Kumasi

The climate is typically tropical, with total annual rainfall of about 1400 mm, the average maximum and minimumtemperatures are about 35C and 23C which occurs in February and August respectively [10]. The potentialevapotranspiration (PET) is estimated at 1450mm/y. The average humidity during the wet season is typically high(86%) and falls to about 57% in the dry period [11]. The major rivers and streams in Kumasi include Subin,Wewe, Susan, Aboabo, Oda, Owabi, Suntreso, Akrubu, Acheamponmene and Asuoyeboa.

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Revision of the Rainfall Intensity Duration Frequency Curves for the City of Kumasi-Ghana Journal ..

2.2 Data Collection

The data used in this study was obtained from the Ghana meteorological Services Department. The data consistsof annual maximum series (AMS) of rainfall depth over a period of twenty-two years for nine (9) laps of time:12min, 24min, 42min, 1hour, 2hours, 3hours, 6hours, 12 hours, and 24 hours.

Table 1: Summary of Data Gathered

2.3 Development of IDF Curves

The data set was subjected to frequency analysis to determine the distribution of which best characterize the dataset. The frequency of the rainfall is usually defined with reference to the annual maximum series, which consistsof the largest values observed in each year. The annual maximum series were found to be drawn from the Gumbeldistribution whose parameters were computed by fitting the statistics to the data. The fitting involves calculatingthe 1, 2, 5, 10, 20, 30 and 50 return intervals for each duration. Frequency precipitation PT (in mm) for eachduration with a specified return period Tr (in year) is given by:

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Revision of the Rainfall Intensity Duration Frequency Curves for the City of Kumasi-Ghana Journal ..

The IDF Curves for the city of Kumasi were then developed by plotting the rainfall intensity values, I T against

corresponding durations, t d for the different return periods. The IDF Curves for the city of Kumasi is presented infigure 2.

Figure2: IDF curves for Kumasi

The IDF estimates resulting from this study (Figure 2 and Table 3) are in accordance with the generalproperties of IDF curves. Thus for the same return period, high intensities are related to short durations andsecondly, the curves are parallel; they do not cross each other.

3.2 Comparison of New IDF estimates and Existing Dankwa [7] IDF estimates for Kumasi City Comparing the IDF results in this study to the existing Dankwa [7] IDF estimates for Kumasi, there arevariations in the intensities. In order to picture the degree of variation, the new IDF estimates and the existingDankwa [7] IDF estimates and their incremental rate of change are presented in table 3 below.

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Revision of the Rainfall Intensity Duration Frequency Curves for the City of Kumasi-Ghana Journal ..The results obtained indicate that: The New IDF estimates are higher for shorter durations. The percentage increase ranges between 2% and 25%.This might be as a result of low precipitation trends for shorter durations in 1970s. New IDF estimates are lower for longer durations. The percentage decrease ranges between 3% and 49%. This might be as a result of high precipitation trends for longer durations in 1970s. IV. CONCLUSION AND RECOMMENDATIONThis work involves the revision of the Rainfall Intensity Duration Frequency (IDF) Curves for the city of Kumasi.Annual Maximum Rainfall depths of various durations over twenty-two years for Kumasi were subjected tofrequency analysis to determine the distribution of which best characterize the data set. The annual maximumseries were found to be drawn from the Gumbel distribution whose parameters were computed by fitting thestatistics to the data. The Chi-square test and the Kolmogorov-Smirnov test prove the appropriateness of thefitting.The IDF estimates resulting from this work have been compared with the existing IDF curves prepared byDankwa [7]. The results show that for shorter durations (12 min and 24 min), the new IDF Curves give higherintensities for the same return period; the percentage increase ranges between 2% and 25%, whiles for longerdurations (42min, 1 hr, 2hr, 3hr, 6hr, 12hr and 24 hr), the new IDF Curves give lower intensities for the samereturn period with the percentage decrease ranging between 3% and 49% when compared with the existingDankwa [7] IDF Curves. This might be as a result of low precipitation trends for shorter durations and highprecipitation trends for longer durations in 1970s and before.These therefore call for the revision and updating of the existing Dankwa [7] IDF Curves for all the major citiesand towns in Ghana to take into account the effect of climate change.